Ant -- referencing properties

I have an ant file and a property file. Say, for example, the property file contains properties 'a', 'b', and 'c'. The ant file takes command-line arguments, which should match either of the properties in the property file. Using the command-line arguments, I want to get the value of its corresponding property.

If ${input} contains the command-line argument, how do I reference ${input} so that it accesses ${a}? ${input} outputs 'a', and $${input} outputs ${input}, and ${${input}} outputs itself. If a=33, I want to be able to use 'input' so that I can access 33.